Field of the Invention
The present disclosure relates generally to the treatment of respiratory and cardiovascular conditions, and more particularly, to methods and systems for continuous positive airway pressure (CPAP) therapy selectively providing a pressurized flow of breathable gas to a patient utilizing dual pressure sensors at a source and on a ventilation mask.
Description of the Related Art
Mechanical ventilators comprise medical devices that either perform or supplement breathing for patients. Early ventilators, such as the “iron lung,” created negative pressure around the patient's chest to cause a flow of ambient air through the patient's nose and/or mouth into the lungs. However, the vast majority of contemporary ventilators instead use positive pressure to deliver gas to the patient's lungs via a patient circuit between the ventilator and the patient. The patient circuit typically consists of one or two large bore tubes (e.g., 22 mm inner diameter for adults; 15 mm inner diameter for pediatrics) that interface to the ventilator on one end and a patient mask on the other end.
Ventilators may support either a single limb or a dual limb patient circuit. Single limb patient circuits are typically utilized for less acute clinical requirements such as the treatment of obstructive sleep apnea or respiratory insufficiency. In further detail, the single limb patient circuit, as its nomenclature suggests, involves gas flow from the ventilator to the patient and patient mask over a single conduit. The patient inspires fresh gas from the patient circuit, and expires carbon dioxide-enriched gas that is purged from the system through vent holes in the mask.
One particular application of ventilator devices is in the treatment of obstructive sleep apnea (OSA) syndrome, where the patient's upper airway narrows or collapses during sleep. There are repetitive pauses in breathing that may extend in duration up to half a minute. Although some degree of apnea is considered normal, in more severe cases, daytime sleepiness and fatigue may result as a consequence of reduced blood oxygen saturation, as well as constant interruptions to sleep cycles. In order to retain the patient's airway and ensure normal, uninterrupted breathing during sleep, continuous positive airway pressure (CPAP) therapy may be prescribed.
Generally, CPAP involves the application of positive pressure to open the patient's airway to prevent its collapse, as would otherwise occur during apnea. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient's normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway. However, supplying positive pressure flow into the patient during the expiratory phase generates resistance to the patient's breathing efforts, causing discomfort. Furthermore, toward the end of the patient's expiratory phase, flow and pressure in the airway is naturally minimal, such that positive pressure can cause additional discomfort. Notwithstanding the clinician's best efforts to prescribe a CPAP treatment flow rate that minimizes such extraneous pressure augmentation while ensuring the proper splinting of the airway during inspiration, the patient is still subject to higher pressures than needed throughout the breathing cycle.
Partially in response to this deficiency, CPAP systems that varied the pressure augmentation depending on the patient flow, i.e., inspiration or expiration, were developed. One such system is described in U.S. Pat. No. 6,932,084 to Estes, et al., which is understood to involve pressure augmentation during inspiration and pressure relief during exhalation based upon a patient flow estimator. The delivered pressure, which itself is measured and utilized by a pressure controller in a feedback loop to confirm accuracy, is calculated as a function of a constant CPAP prescription pressure and a proportional value of patient flow. A relief or augmentation constant defines the degree thereof, and the patient flow is estimated. The value of the constant is zero during inspiration, thus providing no pressure augmentation and the delivered pressure is equivalent to the base or prescription pressure. During expiration, the value of the constant is non-zero, and the pressure delivered to the patient is the prescription pressure less a proportional amount of the estimated instantaneous patient flow. A signal representative of the delivered pressure drives the blower hardware delivering therapeutic air flow to the patient. Although the method disclosed by Estes, et al. allows the delivered therapeutic pressure to the patient to be tuned to a greater degree, the patient nevertheless experiences discomfort.
Another approach to the issue of excess pressure at the expiration stage is disclosed in U.S. Pat. No. 7,128,069 to Farrugia et al. When a transition from the inspiration phase to the expiration phase is detected, i.e., when the pressure at the patient's mouth begins to drop, the motor that controls the blower is understood to be de-energized and allowed to free-wheel. After the pressure increases back to a predetermined level, which in the Farrugia et al. disclosure is 3 cm H2O, the blower motor is restarted to bring the pressure at the patient's mouth to prescription levels (10 cm H2O). Thus, the temporary stopping of the blower is understood to function as a pressure relief during expiration. However, due to the wind-up and wind-down times associated with starting and stopping an electrical motor, patient comfort is not optimized due to the existence of residual pressure.
Accordingly, there is a need in the art for improved methods and systems for continuous positive airway pressure (CPAP) therapy including the use dual pressure sensors at a source and on a ventilation mask to control an exhalation valve.